Recently active-matrix organic light-emitting diode (AMOLED) displays with amorphous silicon (a-Si), poly-silicon, organic, or other driving backplane have become more attractive due to advantages over active matrix liquid crystal displays (AMLCDs). For example, the advantages include: lower power, wider viewing angle, and faster refresh rate displays.
Currently most of the AMOLED displays use poly-silicon backplanes. However, due to its relative infancy, ongoing processing concerns, and limited available capacity, the usage of the poly-silicon backplanes does not lend itself to low-cost manufacturing.
By contrast, amorphous silicon (a-Si) leverages the vast installed infrastructure of proven AMLCD production, promising much lower manufacturing costs as opposed to that of polysilicon. As well, an a-Si solution exposes the large global base of current liquid crystal display manufacturers to the AMOLEDs, thereby accelerating its introduction commercially.
However the usage of a-Si in AMOLED backplanes encounters two issues, namely low mobility and device instability due to the shift of the threshold voltage of a transistor. The threshold voltage shift poses a design constrain for the AMOLED backplanes.
To overcome these issues, many pixel circuits have been proposed ([Ref. 1] A. Nathan, A. Kumar, K. Sakariya, P. Servati, S. Sambandan, K. S. Karim, D. Striakhilev, “Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic,” IEEE Journal of Solid State Circuits, vol. 39, pp. 1477-1486, 2004, [Ref. 2] J.-C. Goh, J. Jang, K.-S. Cho, and C.-K. Kim, “A new a-Si:H thin-film transistor pixel circuit for active-matrix organic light-emitting diodes,” IEEE Electron Device Lett., vol. 24, no. 9, pp. 583-585, 2003, [Ref. 3] James L. Sanford and Frank R. Libsch, “TFT AMOLED Pixel Circuits and Driving Methods,” SID 2003, pp. 10-13). These circuits can be broadly classified as being either current programmed or voltage programmed.
Despite the accuracy, the current programmed circuits by A. Nathan et al. [Ref. 1] may face a “settling time” problem due to the low transconductance of the a-Si TFT coupled with a high line capacitance.
The voltage programmed circuits by J.-C. Goh, et al. [Ref. 2] and James L. Sanford et al. [Ref. 3] generally do not suffer from this “settling time” problem. However, they require techniques to decrease the dependence of OLED current on the threshold shift of a thin film transistor (TFT).
Numerous other compensation techniques have been introduced. However they either use complex pixel circuits, each having more than 2 TFTs and/or have programming methods which suffer from the same programming time issues as with current programmed circuits.